1. Field of the Invention
The invention relates to a user-addressing method in an authorised access television system in which a transmitter transmits collective address words enabling selection of a group comprising a plurality of special users for each of these words.
The invention also relates to a receiver for enabling a user to receive transmissions in which collective addresses are used, provided with a memory which contains information components defining a plurality of collective addresses which are different from each other but all of which concern said user.
2. Description of the Related Art
For managing the right of access by users in such an authorised access television system, one should be able to select a group of special users so as to address to them a message of controlling the rights of access concerning these users. The addressing mechanisms currently used in the majority of systems allowing conditional access use a set of addresses specifying the receiver which they address. Particular distinction can be made between:
the unique address specifying a sole user, PA1 the shared address specifying, for example the group of clients of a given program operator, PA1 the collective address specifying a group of users each having something in common, for example "the petanque players of Tokyo". PA1 a collective address word has a given overall length which is divided into several sections of equal length, placed end to end, PA1 a collective address is defined by PA1 a collective address word is transmitted by the transmitter in the form of a word having said overall length, whose section corresponding to the compulsory position for this address comprises the field of bits of the address and the rest of which is composed of bits which are all identical. PA1 a word having a length which is equal to said overall length is constructed by placing several fields end to end, each corresponding to said contents of an address taken from said plurality of collective addresses available to the user receiver, each of these fields being placed in the compulsory position defined for the address, PA1 this word is compared with the collective address word of the received message, while conformity between the two words is searched separately for each field, PA1 the message is recognised if one identical field is found.
A user receiver is informed in advance of the definitions of the different collective addresses concerning this receiver, which addresses are registered in a memory. At the receiver end, for recognizing whether an address word received from a transmitter concerns the receiver, it is searched whether this address word contains an address which is identical to one of those registered in the memory. This process is referred to as "filtering" and the means for realising this process are referred to as "filters".
As far as the collective addresses are concerned, several operators of different programs may address users in this way and each operator has his own manner of defining groups. There is a different address for each definite group and each operator has his own addresses; this finally leads to a large number of addresses. The total number of addresses to be treated by a receiver may thus exceed the number of filters which it has for the acquisition of messages.
A known collective address system is specified in the European standard for television scrambling, referred to as "Eurocrypt". In accordance with this standard, an address of twelve bits is defined for a collective address and the selection is realised at the user end by comparison with a word of twelve bits.